Successive biochar amendment improves soil productivity and aggregate microstructure of a red soil in a five-year wheat-millet rotation pot trial

Soil structure affects soil physical, chemical, and microbial processes, and is a key factor influencing soil productivity and ecological functions. Biochar (BC) has a porous structure, large surface area and high organic carbon content and therefore has the potential to regulate soil structure and productivity. In the present study, we evaluated the effects of five-year successive seasonal application of BC at three different rates (0, 2.25 and 22.5 Mg ha(-1)) along with chemical fertilizer on soil microstructure and soil fertility of a red soil using a pot trial under wheat-millet crop rotations. We found continuous application of chemical fertilizers reduced soil fertility with progressive reductions in wheat and millet biomass yields over five rotation cycles. While BC amendment slowed the decline of soil productivity with their yield of grain and straw significantly higher than the treatment without BC. Application of high rate of BC (BC22.5) significantly improved soil fertility, with increased soil pH (3.29 units), cation exchangeable capacity (5.66 cmol kg(-1)), plant available water capacity (50.0%), soil organic carbon (306%), total nitrogen (182%), available phosphorus (241%), potassium (513%), calcium (245%) and magnesium (265%), and significantly decreased bulk density (0.52 g cm(-3)), soil exchange acidity (6.37 cmol kg(-1)) and exchange Al3+ concentrations (5.87 cmol kg(-1)) after five rotations. Specially, BC improved aggregate stability, infra-aggregate pore structure and water retention capacity, manifesting that BC might be an effective soil amendment to reduce soil erosion in this region. Our results indicated that long-term successive seasonal BC application could improve soil fertility and soil aggregation, and thus eliminate soil acidification and restore soil productivity in the acidic red soil.